The present invention relates to an improvement of a control apparatus for an electric car, and particularly to a control apparatus for an electric car driven by an induction motor.
The use of an induction motor for driving an electric car not only improves reliability due to no use of rectified current to the motor, but also improves adhesion property of the electric car because the induction motor has a constant speed characteristic such that, the speed of the motor is kept substantially constant when the frequency of an applied AC voltage is constant. Thus, realization of an induction motor driving electric car system has been expected for a long time.
However, the realization is delayed because of the necessity of a power converter as a polyphase AC power supply capable of widely controlling the frequency and voltage, for example, an inverter or a cycloconverter.
Recently, with the development of semiconductors such as thyristors, gate turn-on thyristors, there has been available a power converter satisfying the above control requirements, and thus an electric car using an induction motor has been put into practical use.
To supply polyphase AC power of variable frequency and variable voltage to an induction motor, a DC to AC converter is mounted on the car in the case of DC electric railway, or an AC to AC converter (cycloconverter) or both AC to DC and DC to AC converters are mounted thereon in the case of AC electric railway.
These power converters take one of the following control systems in the constant torque mode of powering or regenerative braking for accelerating or decelerating the electric car:
(1) a control system in which the ratio (v/f) of the voltage v to the frequency f is made constant and the slip frequency f.sub.s is maintained constant, so that the current I is substantially constant,
(2) a control system in which the slip frequency f.sub.s and the current I are maintained constant, so that, the ratio (v/f) is substantially constant.
In either system, it is necessary to detect a speed frequency corresponding to the rotational speed of the induction motor in order to control the slip frequency f.sub.s constant.
In other words, in the powering operation, the output frequency of the converter is increased by the slip frequency f.sub.s with respect to the rotational speed signal f.sub.f of the induction motor, thereby generating an accelerating torque, and therefore the frequency command f.sub.p to be applied to the power converter is given by EQU f.sub.p =f.sub.f +f.sub.s ( 1)
In the regenerative operation, the output frequency of the converter is decreased by the slip frequency f.sub.s with respect to the rotational speed (f.sub.f) of the induction motor, thereby generating a decelerating torque, and therefore the frequency command f.sub.p to be applied to the power converter is given by EQU f.sub.p =f.sub.f -f.sub.s ( 2)
For detection of this speed frequency f.sub.f, there have been used a speed generator (tachometer generator), a pulse generaor or the like, connected to the induction motor.
Electric cars having induction motors have a remarkable advantage that, as described above, the adhesion property is improved by the constant speed characteristic of the induction motor. That is, as is apparent from the speed-torque characteristic of the induction motor, when the powering torque is too large the powering operation and the driving wheel skids, so that the motor tends to increase the rotational speed, the powering torque decreases, acting to prevent the skid of the wheel. On the other hand, when the braking torque is too large during the regenerative operation and the driving wheel slides, so that the rotational speed of the motor tends to decrease, the braking torque decreases, preventing the slide of the wheel.
However, if the output frequency f of the power converter is made equal to the frequency command f.sub.p determined by equations (1) and (2) by a frequency control system as described previously, a skid or slide once generated can not be suppressed.
The skid at the powering operation will now be considered. When a skid once occurs, increasing the rotational speed f.sub.f of the motor, the frequency command f.sub.p increases according to equation (1). Therefore, the speed-torque characteristic of the induction motor is moved in parallel toward the higher speed range, thus increasing the powering torque and promoting the skid.
Similarly, the slide at the regenerative operation can not be suppressed.
Thus, in the conventional frequency control system, the preferable characteristic of the induction motor is lost in effect by the action of the control system.
In electric cars, of course, there are provided a plurality of induction motors, and a plurality of driving wheels connected thereto, and therefore, like the system used in the conventional DC motor car, the minimum (at the powering operation) or maximum (at the regenerative operation) number of revolutions is used to determine the rotational speed f.sub.f in equation (1) or (2). In this way, even if some of the driving wheels cause skid or slide, this can be suppressed.
If, however, all the driving wheels cause skid or slide, these are promoted contrary to suppression thereof.
The suppression thereof can be achieved by providing an additional control system, but lowering reliability results in addition to increase in the complexity and cost of the system.